1. Field of the Invention
The invention relates to methods and apparatus for monitoring samples and in particular to the monitoring of such samples using nuclear magnetic resonance (NMR) techniques. The invention is primarily concerned with relatively low resolution pulsed NMR systems and should be contrasted with other techniques which operate in frequency selective modes.
2. Description of the Related Art
Nuclear magnetic resonance relies on the fact that when certain nuclei are exposed to a uniform static magnetic field their magnetic vectors will precess around the axis of the applied magnetic field with a frequency proportional to the strength of the field. This frequency is known as the Larmor frequency and is given by the formula n.sub.o =yH.sub.o where y is the gyromagnetic ratio of the particular nucleus and H.sub.o is the strength of the magnetic field. Alignment of the magnetic moments of the nuclei creates a bulk magnetisation vector in the sample oriented along H.sub.o. If a pulse of rf energy is applied, those nuclei with spins precessing with the frequency or frequencies of the pulse can absorb the rf energy and information about the nuclei can be obtained from the subsequent relaxation and release of this energy.
Oxford Instruments manufactures and sells a product known as the QP20 which can analyse samples at a bulk level to determine properties such as bulk nuclear density, chemical characteristics such as moisture content and oil content, and physical characteristics such as viscosity and solid/liquid ratio of constituents. The concept of solid/liquid ratio determination extends to cover the discrimination of physically distinct phases where the relaxation times may be characterised as "fast" and "slow", relative to one another, typically with an order of magnitude difference. It is known to apply a single rf pulse which rotates the bulk magnetic vector of the sample by 90.degree. from the equilibrium position (referred to as a 90.degree. pulse) and to monitor the resulting Free Induction Decay (FID) signal and obtain information relating to the fast and slow relaxing phases of the sample in the form of a ratio. These phases will henceforth be defined as `solid` and `liquid` respectively although as stated previously, these descriptions may not be interpreted literally and are intended to define a relative relationship. In one example a fast relaxation rate will be less than 10 .mu.s and a slow relaxation rate greater than 100 .mu.s corresponding to "solid" and "liquid" phases respectively.
Two examples of systems which may be distinguished in this way are given below, with approximate relaxation rates identified:
______________________________________ "Fast" or "Solid" "Slow" or "Liquid" ______________________________________ Ice (appx. 10 us) Pure Water (around 500 mS) Crystalline Polyethylene Amorphous polyethylene (&lt;10 uS) (appx. 35 us) ______________________________________
It will be noted that the relaxation rate of amorphous polyethylene is less than 100 .mu.s but in the context of amorphous polyethylene/crystalline polyethylene this is relatively slow.
The problems with this process are firstly that the component of the FID due to `liquid` varies significantly with the inhomogeneity of the main magnetic field applied to the sample, and secondly the component due to `solid` decays very rapidly in comparison with typical instrument measurement times.